1. Field of the Invention
The present invention relates to an optical wavelength converter module for converting a fundamental generated by a light source to a second harmonic having a wavelength which is 1/2 of the wavelength of the fundamental, with an optical wavelength converter device.
2. Description of the Prior Art
Various attempts have heretofore been made for converting the wavelength of a laser beam into a shorter wavelength based on the generation of a second harmonic by a nonlinear optical material. One example of an optical wavelength converter device for effecting such laser wavelength conversion is a bulk crystal type converter device as disclosed, for example, in "Introduction to Optical Electronics" written by A. Yariv and translated by Kunio Tada and Takeshi Kamiya (published by Maruzen K. K.), pages 200-204. This optical wavelength converter device relies upon the birefringence of a crystal in order to meet phase matching conditions. Therefore, any material which does not exhibit birefringence or exhibits only small birefringence cannot be employed even if it has high nonlinearity.
A fiber type optical wavelength converter device has been proposed to solve the above problem. The optical wavelength converter device of this type is in the form of an optical fiber comprising a core made of a nonlinear optical material surrounded by cladding. One example of such an optical fiber is shown in the bulletin Vol. 3, No. 2, of the Mirooptics Research Group of a Gathering of the Applied Physics Society pages 28-32. Recently, many efforts are directed to the study of a fiber type optical wavelength converter device since it can easily gain matching between a fundamental and a second harmonic.
For increasing the wavelength conversion efficiency of the fiber type optical wavelength converter device, it is preferable to employ an optical material of high nonlinear optical constants as a core. Known optical materials of high nonlinear optical constants include MNA(2-methyl-4-nitroaniline) disclosed in Japanese Laid-Open Patent Publication No. 60-250334, NPP(N-(4-nitrophenyl)-L-prolinol), NPAN(N-(4-nitrophenyl)-N-methylaminoacetonitrile), and the like disclosed in J. Opt. Soc. Am. B Vol. 4, page 977 (1987).
Where a fiber type optical wavelength converter device is constructed of a nonlinear optical material of the type described above, however, the wavelength conversion efficiency of the optical wavelength converter device is not so increased since the crystal is not oriented in such a direction as to be able to utilize the maximum nonlinear optical constants of the material.
The wavelength conversion efficiency of the optical wavelength converter device is longer as the length of the device is larger. The nonlinear optical materials referred to above are however not suitable for making long optical wavelength converter devices because it is difficult to obtain a uniform monocrystal from those nonlinear optical materials.